在使用Altium Designer中，会经常遇到某个元件对应多个PCB封装，通常包含后缀L、M、N三种形式，如FM33G048元件对应3个PCB封装，分别是LQFP80_L、LQFP80_M和LQFP80_N。那这三个有什么区别呢？

其实这些封装是根据PCB电路板的空间密度进行区分的，其中

L表示Least Use Environment（Level C - High Density，高密度）

M表示Most Use Environment （Level A - Low Density，低密度）

N表示Nominal Use Environment（Level B - Medium density，中密度）

通常情况下，使用中密度N就可以。如果电路板空间比较紧凑，元件排列密集，则采用高密度L形式。

在IPC7351标准中，对这些情况进行了详细描述：

Land Pattern Information
Density Level A: Maximum (Most) Land Protrusion - For low-density product applications, the 'maximum' land pattern condition has been developed to accommodate wave or flow solder of leadless chip devices and leaded gull-wing devices. The geometry representing these devices, and inward and ''J''-formed lead contact device families, may provide a wider process window for reflow solder processes.

Density Level B: Median (Nominal) Land Protrusion - Products with a moderate level of component density may consider adapting the 'median' land pattern geometry. The median land patterns furnished for all device families will provide a robust solder attachment condition for reflow solder processes and should provide a condition suitable for wave or reflow soldering of leadless chip and leaded gull-wing type devices.

Density Level C: Minimum (Least) Land Protrusion - High component density typical of portable and hand-held product applications may consider the 'minimum' land pattern geometry variation. Selection of the minimum land pattern geometry may not be suitable for all product use categories. The use of classes of performance (1, 2, and 3) is combined with that of component density levels (A, B, and C) in explaining the condition of an electronic assembly. As an example, combining the description as Levels 1A or 3B or 2C, would indicate the different combinations of performance and component density to aid in understanding the environment and the manufacturing requirements of a particular assembly.